How can I debug disappearing emails?

[eluser]rideearthtom[/eluser]
I have built an app which sends a fair few emails using the CI email class. $this->email->send() returns true, but the email is never received by the recipient. I've tried mail, sendmail, text, html, all config options I can think of.

I have full access to the server, so is there a way of seeing any logs of the underlying PHP activity or mail/sendmail activity?